Livestock Research for Rural Development 21 (10) 2009 Guide for preparation of papers LRRD News

Citation of this paper

Comparative potential nutritive value of grasses, creeping legumes and multipurpose trees commonly in sub humid region in the Eastern parts of Tanzania

D J Mtui1, F P Lekule2, M N Shem2, T Ichinohe2and T Fujihara1,3

1Laboratory of Animal Science, Faculty of Life and Environmental Science, Shimane University, Matsue, Shimane 690-8504, Japan
dorahmtui@hotmail.com
2Department of Animal Science and Production, Sokoine University of Agriculture, P.O. Box 3004, Morogoro, Tanzania
3Faculty of Bioresource, Mie University, Tsu-shi, Mie 514-8507, Japan

Abstract

The nutritive values of three grasses (Cynodon plectostychus, Panicum maximum, Pennisetum purpureum), three creeping legumes (CL) (Calopogonia muconoides, Macroptilium atropurpureum, Neonotonia wightii) and three multipurpose trees (MPTs) (Leucaena leucocephala, Gliricidia sepium, Morus alba) native to Eastern Tanzania, collected during two seasons (i.e. wet and dry) were evaluated. Forages were analyzed for chemical composition including polyphenolics and in vitro gas production characteristics. Tannin bioassay studies were done on CL and MPTs after in vitro incubation with or without addition of polyethylene glycol (PEG) as a tannin binding agent.

 

The forages had variable crude protein contents which were influenced by seasonal variations. The CP content ranged from 120 (Cynodon plectostychus, wet season) to 69 g/kg DM (Pennisetum purpureum, dry season) in grasses; from 217 (Macroptilium atropurpureum, dry season) to 141 g/kg DM (Neonotonia wightii, wet season) in CL and from 317 (Leucaena leucocephala, dry season) to 212 g/kg DM (Morus alba, wet season) in MPTs. Forage type and seasonal interaction affected neutral detergent fiber (NDF) content but not acid detergent (ADF) and acid detergent lignin (ADL) levels. Grasses had high levels of NDF and ADF while MPTs had the lowest levels in each season. The ADL contents were highest in CL but lowest in grasses in both seasons. The contents of total extractable tannins (TET) in CL and MPTs were influenced by the type of season and species interaction and ranged from 67.9 (Neonotonia wightii, dry season) to 9.3 mg/g DM (Morus alba, wet season). The season variation had no effect (P>0.05) on the in vitro gas production volumes and fermentation characteristics. At 12h of incubation, grasses produced least gas compared to CL and MPTs. From 24h to 72h of incubation, Calopogonia muconoides produced highest volume of gas. At 96 h of incubation, Cynodon plectostychus produced the highest gas volume whilst Neonotonia wightii produced least gas volume throughout incubation period. The potential gas production was highest (P<0.05) in grasses compared to other forages and the high rate of gas production was observed in MPTs. Addition of PEG improved (P<0.05) 24h gas production, organic matter digestibility (OMD) and metabolisable energy (ME) in CL and MPTs and both in wet and dry seasons. In both seasons, high levels of 24 h gas production, OMD and ME were observed in Calopogonia muconoides. The lowest responses were in Gliricidia sepium.

 

Results indicate that seasonal variation affected protein content but not in vitro fermentation characteristics of the studies species. Creeping legumes (CL) and MPTs were observed to have high CP content (>100 g/kg DM) and moderate NDF levels (< 550 g/kg DM) suggesting their potential to be used as protein supplements. Polyethylene glycol (PEG) treatment improved the digestibility of Neonotonia wightii and Leucaena leucocephala (dry season) previously depressed by tannin’s anti nutritive activity, thus, improves their nutritive potential.

Key words: chemical composition, forages, in vitro gas production, season


Introduction

Natural pastures species are the main source of feed for ruminants, which form the major component of domesticated livestock in Tanzania. However, bimodal distribution of rainfall, uneven seasonal growth and availability of pasture has been considered as the major limitations to constant supply of forage for ruminants. When available even in limited quantity, grasses can not maintain the animal much of the year. The low quality of grasses is reflected in low production and reproductive performance, as well as slow growth in ruminants (Mafwere and Mtenga 1990; Ndemanisho et al 1998; Kakengi et al 2001).

 

Forage legumes are utilized in tropical and sub-tropical to ruminants grazing poor quality roughages especially during dry season and their importance in ruminant production has been recognized in Tanzania. They are well known to improve the efficiency of protein digestion in rumen and have been incorporated into feeding regime to improve the nutrition status of the animals (Mero and Uden 1998; Kakengi et al 2001; Rubanza et al 2007). Tropical leguminous forages have been reported to contain tannins in varying concentrations. The presence of tannins in these legumes may reduce the availability of nitrogen and amino acids required for rumen microbial growth due to formation of strong tannin-protein complexes. When in high concentration in the leguminous forages, tannins reduce ruminal and postruminal digestion of protein (Barry et al 1986). However, when the tannins concentrations are low, they have been shown to prevent extensive proteolysis in the rumen and increase intestinal absorption of amino acids (Barry et al 1986).

 

There is scant information on the nutritive value of local forages found in Tanzania, especially sub humid areas in Morogoro Region (Mtui et al 2008). In addition, effect of seasons on the chemical composition and nutritional value of leguminous forages is largely unknown. The use of chemical composition in combination with in vitro digestibility and degradability can be a useful tool for preliminary evaluation of nutritive value of feed resources.

 

Therefore, the current study was conducted to establish the effect of season on the chemical composition and in vitro fermentation characteristics of the grasses, creeping legumes and multipurpose trees found in the sub-humid areas of Tanzania, and to examine the effect of addition of polyethylene glycol on the in vitro digestibility of the leguminous forages.  

 

Material and methods 

Study area and collection of forages

 

Forages comprised of grasses; Cynodon plectostychus, Panicum maximum, Pennisetum purpureum, creeping legumes (CL): Calopogonia muconoides, Macroptilium atropurpureum, Neonotonia wightii and multipurpose trees (MPTs): Leucaena leucocephala, Gliricidia sepium, Morus alba, collected from four administrative wards in Turiani division (370 36’- 380 E; 50-70 S), Mvomero district, Morogoro, Tanzania  during wet (March-May) and dry (July-September) seasons, were used for this study. The area lies approximately 380-500 m above sea level and is sub-humid, receives bimodal rainfall which range between 1000-1200 mm per annum, with most of rains falling between February and May. Average temperatures range from 150 C to 29 0C per annum. The area is composed mainly of lowland and small hills covered with grass intermingled with shrubs and leguminous plants.

 

Each season, sampling was done early in the morning when farmers went out to harvest/ cut forages to feed their animals under “cut and carry” system. Grasses and CL were collected from 10 subplots randomly selected using a quadrant technique. In each plot a metal quadrant (0.5m2) was thrown at the interval of 5 m in 16 linear transects of about 35 m each. Samples were harvested about 5 cm above the ground using a hand sickle until the adequate amounts were obtained. A total of 10 samples were collected from each sub locations.

 

Multipurpose trees samples were harvested from 20 randomly selected farms in each ward where these trees were planted along the edges as farm boundaries, alley cropping and or inter-cropped trees in crop farms. On each farm, with a random start, leaves and twigs were plucked from 10-12 trees.

 

Samples of individual species were dried at 50░C using an airforced oven for 48 h to constant weight. Then, all samples of each species type were pooled and ground to pass through 2.0 mm sieve. Thereafter, the sample was sub-sampled into 27 samples, which represented three (3) samples per species per season.

 

Chemical analyses

 

Crude protein (N x 6.25, Kjeldahl technique) were measured according to standard methods (AOAC, 1984). Neutral detergent fiber (NDF), acid detergent fiber (ADF) and acid detergent lignin (ADL) were determined according to the methods of Van Soest et al (1991). The extraction of phenolics was done using 70% aqueous acetone. The concentration of total extractable phenolics (TEPH) was assayed using Folin Ciocalteu and tannic acid standard (Makkar 2000). Total extractable tannin (TET) was estimated indirectly after being absorbed to insoluble polyvinyl polypyrrolidone (PVPP). The concentration of TET was calculated by subtracting the TEPH remaining after PVPP treatment from TEPH. For both the TEPH and TET, the absorbance was measured using a spectrophotometer (Quatification-2 program pack, Shimadzu Corporation, Kyoto, Japan).

 

In vitro organic matter degradability study

 

Animals and rumen fluid extraction


Rumen liquor for the in vitro gas production and tannin bioassay was obtained from three mature healthy Japanese Corriedale sheep fitted with permanent rumen fistula (70 mm). The animals were fed on standard diet of 800 g DM timothy hay and 200 g DM concentrate (2-parts wheat bran and 1-part rolled barley) twice daily at 0900 and 1600 h in equal sized meals. The animals had free access to water and mineral lick throughout the experimental period. The rumen liquor was withdrawn prior morning feeding, mixed, strained through four layers of cheesecloth and kept at approximately 39oC in water bath flushed with CO2 before use and diluted with culture media as described by Menke and Steingass (1988).

 

In vitro gas production

 

Samples were incubated in vitro with rumen fluid in calibrated glass syringes. About 200 mg of feed sample (milled through 1.0 mm sieve) were weighed in duplicate into 100 ml calibrated glass syringes. Vaseline oil was applied to the piston to ease movement and to prevent escape of gas. The syringes were pre-warmed at 39oC for 1 hr before addition of 30 ▒ 0.5 ml of rumen liquor buffer mixture (ratio 1:2) into each syringe. Blanks with buffer rumen fluid without feed sample were also included. All the syringes were incubated in a water bath maintained at 39 ▒ 0.1oC.  The gas readings (ml) were recorded after 3, 6, 12, 24, 48, 72 and 96 h of incubation. 

 

The gas production characteristics were estimated by fitting the mean gas volumes to the exponential equation by ěrskov and McDonald (1979):

 p = a + b (1-e-ct),

Where,

P is the gas production (mlg-1 OM) at time t
a is the gas production (mlg-1 OM) from immediate soluble OM fraction.
Constant b is the gas production (mlg-1 OM) from the insoluble degradable fraction,
a + b is the potential gas production and
c is the gas production (ml/g OM) rate constant per hour (h) from b.

 

Effect of polyethylene glycol (PEG) addition on digestibility:

 

The adverse effects of tannins on in vitro OM digestibility were assessed by incubation of about 500 mg DM of the feed samples with or without 1.0 g Polyethylene glycol (PEG) (molecular weight 6000) (Makkar 2000). The feed samples were incubated in 100 ml glass syringes (Menke and Steingass 1988). The PEG tannin bioassay was conducted according to Makkar (2000). The syringes were pre-warmed at 39oC for 1 h before addition of 40 ▒ 0.5 ml rumen-liquor buffer mixture (ratio 1:3) into the syringes and incubated in triplicate in a water bath maintained at 39 ▒ 0.1oC. Blanks were also included in the incubation. The gas production readings (ml) were recorded after 2, 4, 6, 8, 12, 16 and 24 h of incubation for both PEG, non PEG treated and blank samples.

 

Feed organic matter digestibility (OMD) (%) and metabolizable energy (ME) (MJkg-1 DM) were estimated from the following Menke and Steingass (1988) and Makkar and Becker (1996) equations based on 24 h gas production (Gv, ml) and crude protein content (CP, % DM):

OMD (%) = 14.88+0.889 Gv +0.45 CP

ME (MJkg-1 DM) = 2.20 +0.136 Gv + 0.057 CP

 

Statistical analysis

 

Gas production data were fitted to the asymptote exponential model using Neway Excel computer program (Chen X.B., Rowett Research Institute, Aberdeen). Analysis of variance (ANOVA) was carried out on chemical composition, phenolics, in vitro gas production characteristics, degradability and digestibility estimates data using the General Linear Model (GLM) procedure (SAS/ Statview 1999). Means on the estimated parameters were compared using Fisher’s least significance difference.

 

Results and discussion

Chemical composition

 

Table 1 shows variation of chemical components analyzed in the forages.


Table 1. Chemical composition of forages collected from Mvomero District, Tanzania

Season

Forage species

g/kg DM

mg/g DM

CP

NDF

ADF

ADL

TEPH

TET

Wet

Cynodon plectostychus

120bc

656ab

468a

55b

Na

Na

 

Panicum maximum

109bc

775a

482a

37b

Na

Na

 

Pennisetum purpureum

110bc

659ab

446a

49b

Na

Na

 

Calopogonia muconoides

169b

559b

422a

104a

18.1ab

19.4ab

 

Macroptilium atropurpureum

180b

585ab

457a

103a

18.2a b

20.6ab

 

Neonotonia wightii

141bc

564ab

435a

132a

30.8a

32.7a

 

Gliricidia sepium

212b

455bc

305bc

138a

18.1ab

19.1ab

 

Leucaena leucocephala

237ab

465bc

274c

96a

29a

30.6a

 

Morus alba

222ab

384c

268c

58b

8.8b

9.3b

Dry

Cynodon plectostychus

77f

803aa

453a

64b

Na

Na

 

Panicum maximum

82f

701a

466a

44cd

Na

Na

 

Pennisetum purpureum

69f

683ab

378ab

36cd

Na

Na

 

Calopogonia muconoides

186d

576b

379ab

111a

25.4c

27.6c

 

Macroptilium atropurpureum

217c

494bc

332bc

64bc

15.6d

19.8d

 

Neonotonia wightii

175d

540b

387ab

87ab

62.5a

67.9a

 

Gliricidia sepium

256b

489bc

271bc

110a

17.7d

19.1d

 

Leucaena leucocephala

317a

407bc

257bc

85ab

39.6b

42.3b

 

Morus alba

232c

379c

231c

33d

10e

11e

SEM

 

1.41

20.45

14.34

5.16

3.2

3.55

species

 

***

ns

***

***

***

***

Season

 

ns

***

**

**

*

*

Season x species

 

*

*

ns

ns

***

***

SEM: Standard error of the means; For each column and each season, means with different superscripts are significantly different (P<0.05); CP: Crude protein, NDF: Neutral detergent fiber; ADF: Acid detergent fiber; ADL: Acid detergent lignin; TEPH: Total extractable phenolics; TET: Total extractable tannin; na: not analyzed; ns: not significant (P>0.05); *: significant (P<0.05); **: significant (P<0.01); ***: significant (P<0.001)


The crude protein (CP) concentrations varied (P<0.05) among species and between seasons. Among grasses, the high CP levels were recorded in Cynodon plectostychus (120 g/kg DM, wet season) and the lowest levels in Pennisetum purpureum (69 g/kg DM, dry season). Creeping legumes and MPTs had high levels of CP in dry season (217 vs. 317 g/kg DM in Macroptilium atropurpureum and Leucaena leucocephala, respectively) and lowest levels in wet season (141 vs. 212 g/kg DM in Neonotonia wightii and Gliricidia sepium, respectively). The fiber components (NDF, ADF and ADL) varied significantly (P<0.05) between seasons but with exception of NDF, the concentration levels in ADF and ADL didn’t show any interaction between forage species and the type of season. Panicum maximum, Macroptilium atropurpureum and Leucaena leucocephala had highest NDF and ADF levels in wet season while Cynodon plectostychus, Calopogonia muconoides and Gliricidia sepium had the highest concentration in dry season. The ADL levels were lowest in grasses in both seasons and highest in CL.

 

The TEPH and TET concentrations varied (P<0.05) among species and between seasons. The TEPH and TET concentrations in wet season, ranged from 8.8 to 30. 8 mg/g DM and 9.3 to 32.7 mg/g DM, in Morus alba and Neonotonia wightii, respectively.  During dry season, the TEPH and TET concentrations ranged from 10 to 62.5 mg/g DM and 11 to 67.9 mg/g DM in Morus alba and Neonotonia wightii, respectively.

 

Dry season is the most difficult period in ruminant production in the tropics as grasses, which form basal diet tend to have low protein contents. Protein is the most limiting nutrient for ruminant productivity, a deficiency being manifested in overall low performance of the animal. Therefore, any feed offered to animal should provide enough nitrogen for the microbes in the rumen for optimal animal performance.

 

The protein content of CL (140-216 g CP/kg DM) is consistent with values reported by Mero and Uden (1998) for Tanzanian herbaceous legumes; and those reported elsewhere (Matizha et al 1997) in the tropical region. The observed high CP content of the MPTs (211-317 g/kg DM) justifies the use of the MPTs to supplement poor quality natural pastures (Kakengi et al 2001; Rubanza et al 2007). Low CP contents in CL and MPTs in wet season compared to the ones in dry season is partly explained by the proportion of leaves/herbage harvested for the purpose of the study. During dry season, only the greenest parts of the forages were harvested. Therefore, leguminous forages represent potential feed resources that could be utilized as protein supplement to correct deficient nitrogen to ruminant animals fed on poor quality basal roughages especially during dry seasons.

 

With the exception of grasses, CL and MPTs were observed to be of good nutritional quality due to high CP content (>100 g/kg DM) and moderate NDF content (< 550 g/kg DM) (Leng 1990). High fiber content of grasses was not uncommon as grasses are known to have high fiber content even in wet season compared to other feeds (Pamo et al 2007). The NDF content of more than 65 % NDF was reported to limit DM intake (Van Soest 1991) thus they are of low quality. Therefore, moderate fiber compositions (NDF, ADF and ADL) of the CL and MPTs indicate a highly promising nutritive potential.

 

 The tannin levels (with exception of Neonotonia wightii and Leucaena leucocephala in dry season) in CL and MPTs were less than 30 mg/g DM. This is the range that has been reported to have nutritional benefits to ruminants by protecting dietary proteins from excessive ruminal degradation without affecting feed intake or fiber digestion. Levels higher than 40 mg/g DM depress intake and animal growth (Barry et al 1986; Jackson et al 1996). High tannin levels in tropical leguminous forages had previously reported (Jackson et al 1996; Mupangwa et al 2000). Therefore, with the exception of Neonotonia wightii and Leucaena leucocephala, it is unlikely that tannin in the remaining forages would have detrimental effects on feed intake and growth of ruminants.

 

In vitro gas production and fermentation characteristics

 

The in vitro cumulative gas production levels and their fermentation characteristics did not vary (P>0.05) between wet and dry season therefore the reported data are the average values and are shown without seasonal variations (Table 2).


Table 2. In vitro cumulative gas production (mg/g OM) and degradability characteristics; a+b (ml/g OM) and c rate of gas production (ml per h)

Species

Cumulative gas production

a+b

c

12 h

24 h

48 h

72 h

96 h

Cynodon plectostychus

15.7c

27.2b

37.0ab

40.9a

44.0a

218a

0.042a

Panicum maximum

15.7c

26.7b

34.9ab

39.4ab

43.2a

229a

0.058a

Pennisetum purpureum

15.9c

27.4b

34.9ab

38.9abc

43.1a

233a

0.037a

Calopogonia muconoides

27.5a

34.7a

39.5a

42.0a

43.4a

46.0b

0.045a

Macroptilium atropurpureum

21.0b

28.9ab

33.2b

35.8bc

37.1bc

45.4b

0.029a

Neonotonia wightii

17.9bc

25.7b

33.5ab

34.8bc

35.6c

44.8b

0.017a

Gliricidia sepium

26.5a

34.1a

37.2ab

38.3ab

39.3bc

51.3b

0.075a

Leucaena leucocephala

27.0a

32.5a

37.4a

39.1ab

40.8ab

43.5b

0.079a

Morus alba

26.1a

32.2a

36.8ab

38.9abc

40.3abc

41.6b

0.06a

SEM

0.65

0.76

0.81

0.9

1.0

15.6

0.05

Species

***

***

***

***

***

***

***

SEM: Standard error of the means; For each column  means with different superscripts are significantly different (P<0.05);  ***: significant (P<0.001)

a+b: the potential gas production c: the rate constant of gas production (ml/g OM) per hour (h) 


However, significant (P<0.05) variations in gas production and fermentation parameters were observed among species. After 12 h of incubation grasses produced least gas compared to CL and MPTs. From 24 h to 72 h of incubation, Calopogonia muconoides followed by grasses produced highest gas volume. At 96 h of incubation Cynodon plectostychusproduced highest gas volume. Throughout incubation period, Neonotonia wightii produced the lowest gas volume. The potential gas production was highest in Pennisetum purpureum and lowest in Morus alba. The rate of gas production was highest in Leucaena leucocephala and Gliricidia sepium and lowest in Neonotonia wightii and Macroptilium atropurpureum

 

Gas production is a result of feed fermentation in vitro that simulates the rumen degradability phenomenon. Gas is produced from feed fermentation and indirectly from CO2 released from buffer mixture by volatile fatty acids (acetates, butyrates and propionates). Differences of feed degradability are largely dependent on the content of fiber and crude protein (Rubanza et al 2003).

 

In the current study, the observed high extent of cumulative gas production volume in grasses than in leguminous forages had been previously reported for the tropical forages (Melaku et al 2003; Osuga et al 2006). The high extent of gas production in grasses might be due to high OM availability in grasses than in CL and MPTs, with the OM fermented to form VFAs and therefore high gas volumes being produced. The high CP and relatively higher non-structural carbohydrate contents of CL and MPTs may have resulted into the substrate yielding more microbial biomass production and polysaccharides storage in the cell than those of the grasses and hence, causing less gas production (Osuga et al 2006).

 

The observed low gas production in Neonotonia wightii compared to other forages might be due to its high tannin content. Tannins depress forage digestibility by binding feed nutrients rendering them unavailable for digestion. High tannin content in the feed reduces the population of fiber degrading bacteria in the rumen and hence low activity (Makkar and Becker 1996; Getachew et al 2000). The high gas production observed in Leucaena leucocephala despite presence of tannins has been previously reported (Norton 1994). According to Norton (1994), sometimes tannins do not reduce digestibility, which is probably due to the nature and chemical properties of tannins in those forages, indicating that not all tannins decrease protein degradability in the rumen.

 

Based on gas production potential in forages, Morus alba ranked the lowest while grasses were the highest. This trend could be due to amount of lignin and extent of lignifications and possibly due level of tannins and other anti nutritive factors and their related biological activities. The rate at which different feed sample are fermented is a reflection of microbial growth and accessibility of feed to microbial enzymes (Getachew et al 2000). High rates of feed OM degradability observed in MPTs in this study had been previously reported. Makkar and Becker (1996) and Keir  et al (1997) reported values of 0.026 – 0.059 for MPTs which are similar to those obtained in the present study. This is probably due to presence of high fermentable carbohydrate, especially at initial incubation hours. Thus the higher values obtained for the rate of feed degradability in the forages may indicate a better nutrient availability for rumen microorganisms in animals fed with these forages (Getachew et al 2000).

 

Effect of polyethylene glycol (PEG) on in vitro digestibility

 

The effect of addition of PEG on 24h gas production, OMD and ME are presented in Table 3.


Table 3.  Effect of PEG on in vitro digestibility

Season

Species

24h gas production volume, ml

OMD, %

ME, MJ/kg DM

-PEG

+PEG

Incr

-PEG

+PEG

Incr

-PEG

+PEG

Incr

Wet

Calopogonia muconoides

64.5c

69.2b

7.32

148bc

152bc

2.75

20.6bc

21.2bc

3.11

 

Macroptilium atropurpureum

75.4ab

79.9a

5.95

163b

167b

2.39

22.7bc

23.3b

2.68

 

Neonotonia wightii

60.6c

61.3c

1.06

132c

133c

0.43

18.5c

18.6c

0.49

 

Gliricidia sepium

83.4a

85.7a

2.7

184ab

186ab

1.07

25.6a

25.9ab

1.21

 

Leucaena leucocephala

70.1bc

71.1b

1.38

197a

198a

0.43

27.0a

27.1a

0.42

 

Morus alba

75.3ab

79.9a

6.03

182ab

186ab

2.18

25.1ab

25.7a

2.47

Dry

Calopogonia muconoides

73.0b

78.9b

8.14

163d

169d

3.23

22.7c

23.5c

3.57

 

Macroptilium atropurpureum

61.4bc

65.4c

6.85

172d

176cd

2.08

23.5bc

24.1c

2.34

 

Neonotonia wightii

66.6b

71.3c

7.15

153de

157e

2.77

21.2c

21.8d

3.02

 

Gliricidia sepium

85.6a

86.2a

0.78

206b

206b

0.29

28.4a

28.5a

0.32

 

Leucaena leucocephala

66.6b

67.9c

2.06

217a

218a

0.56

29.3a

29.5a

0.61

 

Morus alba

72.9b

73.3bc

0.56

184c

184c

0.2

25.3ab

25.4bc

0.24

SEM

 

3.77

4.64

 

6.59

6.67

 

0.81

0.86

 

Species

 

 ***

 ***

 

***

***

 

***

***

 

Season

 

 ***

 ***

 

***

***

 

***

***

 

Season x species

 

 **

 **

 

*

**

 

*

**

 

SEM: Standard error of the means; For each column and each season, means with different superscripts are significantly different (P<0.05); *: significant (P<0.05); **: significant (P<0.01); ***: significant (P<0.001); % increase =  (+ PEG gas volume (ml) – - PEG gas volume (ml)) X 100/ - PEG gas volume (ml)


Although the type of season did not influence the rate of cumulative gas production, it affected (P<0.05) the 24h gas production, OMD and ME when treated with PEG. The highly seasonal variation was noted in Neonotonia wightii (1.06 vs. 7.15 %, 0.43 vs. 2.77 %, and 0.49 vs. 3.02 %, for wet and dry season on 24 h gas production, OMD and ME, respectively). Among MPTs, M. alba had the highest and lowest increase in 24 h gas production, OMD and ME (6.03 vs. 0.56%, 2.18 vs. 0.2%, and 2.47 vs. 0.24%, for wet and dry season on 24 h gas production, OMD and ME, respectively).

 

Overall, CL and MPTs in the current study had moderate to low increase in gas production (<10%) on addition of PEG indicating that PEG’s effect and/or tannin content is low. The lower effect supports the concept that the nature and magnitude of the effect of PEG depend on factors such as tannin structure and level of tannin in the foliage. PEG increases the availability of nutrients to rumen micro-organisms, especially carbohydrates and nitrogen (Silanikove et al 1999).

 

In the present study, the highest increased of in vitro gas production, OMD and ME upon the PEG treatment was observed in Calopogonia muconoides in both seasons. This may partly be explained by the presence of other anti-nutritive factors apart from tannin, which were neutralized by PEG (Silanikove et al 1999). The high increase of responses upon PEG treatment in Neonotonia wightii emphasizes the negative effect of tannins on digestibility. PEG, a non-nutritive synthetic polymer, has a high affinity to tannins and makes tannins inert by forming tannin PEG complexes (Makkar et al 1995). PEG also can also liberate protein from the preformed tannin-protein complexes (Barry et al 1986).

 

Some studies clearly showed that PEG supplementation increased the gas production and volatile fatty acid production (Getachew et al 2001; Getachew et al 2002). Current findings of the effect of PEG on improved digestibility and ME of MPTs are considerably lower than those obtained by Rubanza et al (2007), Kamalak et al (2006) and Mtui et al (2008) for tropical browses. Leaves of leguminous trees did not give the same response with PEG treatment, possibly due to differences in chemical composition of tannins (Kamalak et al 2006), variation in tannin anti nutritive activity between foliage species (Rubanza et al 2003), the nature, chemical structure  and degree of polymerization of tannin (Schofield et al 2001).

 

Conclusions 

 

Acknowledgment  

Financial aid to the first author provided by Bishop Williams Memorial Funds (Japan) is deeply acknowledged.

 

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Received 3 March 2009; Accepted 10 June 2009; Published 1 October 2009

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